How-To:: March 2020

Friday 20 March 2020

Slf4j NoClassDefFoundError when compiling SparkJava sample project

Problem:

When trying out the SparkJava demo for websockets (spark-websockets, "Using WebSockets and Spark to create a real-time chat app"), I get the following compilation error:

Exception in thread "main" java.lang.NoClassDefFoundError: org/slf4j/LoggerFactory
 at spark.Service.<clinit>(Service.java:56)
 at spark.Spark$SingletonHolder.<clinit>(Spark.java:51)
 at spark.Spark.getInstance(Spark.java:55)
 at spark.Spark.<clinit>(Spark.java:61)
 at Chat.main(Chat.java:19)
Caused by: java.lang.ClassNotFoundException: org.slf4j.LoggerFactory
 at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
 at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
 at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349)
 at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
 ... 5 more

Workaround:

The following workaround worked as a quick way to try out the demo code:

Open the project's pom.xml file

Add the following dependency:


<dependency>
  <groupId>org.slf4j</groupId>
  <artifactId>slf4j-api</artifactId>
  <version>1.7.21</version>
  <scope>compile</scope>
</dependency>

If this worked, the project should now compile properly.

Notes:

The above workaround is one of many solutions that can work towards a successful compile. Others include adding a JAR file for a logger, for example. This solution was chosen as it is the quickest for a beginner to do while learning how to use SparkJava or while learning programming in Java.

This how-to was verified to work in revision 8cc09e8cea9257a0df3449106a57ad0467faf39b (Sep 4, 2017) of the spark-websockets project. Your results may vary for other revisions.

References:

Thursday 19 March 2020

Meteor VSCode esversion 6 jslint error

Problem:

When using Visual Studio Code to edit my Meteor project, I keep seeing the error:

'import' is only available in ES6 (use 'esversion: 6'). (W119) jshint(W119)

For example:

Workaround:

To hide the jshint errors: in the main folder of your Meteor project, if the file named ".jshintrc" does not exist then add it
In this .jshintrc file, add the following text:
{ "esversion": 6 }

You may need to close and reopen the project in VSCode. If the workaround is successful, you should no longer see the esversion 6 error.

Notes:

This workaround was verified to help remove the jslint errors while using Visual Studio Code version 1.43.1 with Meteor 1.10.1. Your results may vary given different versions. Please also note that this workaround hides the jshint error from being highlighted, but it won't fix compilation errors if your Meteor project isn't properly setup to compile esversion 6.

References:

https://jshint.com/docs/options/

Thursday 12 March 2020

Catalina Jekyll bad interpreter error

Problem:

When trying to run jekyll in Terminal, I get the following error:

zsh: /usr/local/bin/jekyll: bad interpreter: /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby: no such file or directory

I recently upgraded MacOS.

Before you start:

This how-to assumes you already have knowledge of Terminal and the 'sudo' command, given that you've already installed Jekyll in the past. If not, please proceed at your discretion as 'sudo' command can be harmful if used incorrectly.

Solution:

In Terminal, reinstall jekyll via homebrew:


sudo gem install jekyll

Why this works:

The upgrade to Catalina changed key dependencies of Jekyll. While it is possible to manually link them or add them to your PATH, reinstalling Jekyll should provide an easier and more robust solution. You'll likely also need to reinstall any other gems you need.

Disclaimer:

This solution was verified to work in MacOS 10.15.3 with Jekyll 4.0.0. Your mileage may vary with other versions.